1. Field of the Invention
The present invention relates to an apparatus for enhancing the extinction ratio in an optical NRZ-to-RZ converting system using an optical fiber loop mirror together with optical amplifiers.
2. Description of the Prior Art
In all optical network to be developed in the future, various multiplexing techniques and data patterns have be accommodated. That is, an optical transparency has to be maintained between the time division multiplexing and the wavelength division multiplexing, and the data patterns have to be arbitrarily converted.
In this case, particularly, the NRZ (non-return to zero) signals and RZ (return to zero) signals are mostly used, and the NRZ signals can be converted to RZ signals by utilizing the cross gain compression of a semiconductor optical amplifier.
The method of utilizing the cross gain compression has the advantage that the structure is simple, but the extinction ratio is limited to the maximum 8 dB. In order to raise the extinction ratio, high intensity pump signals are required, this being problematic.
This will be described in detail referring to FIGS. 1 and 2.
FIG. 1 illustrates the constitution of the cross gain modulation system to which the present invention is applied. FIG. 2 illustrates the characteristic curve showing the gain of the semiconductor optical amplifier of FIG. 1.
As shown in FIG. 1, the system includes: a first optical polarization adjuster 10 for adjusting the polarization of the NRZ signals and furnishing the NRZ signals to an optical coupler 12; a second optical polarization adjuster 11 for adjusting the polarization of the clock pattern signals and furnishing NRZ optical clock pattern signals to the optical coupler 12; the optical coupler 12 receiving the polarization-adjusted signals from the first and second optical polarization adjusters 10 and 11 to supply them to respective terminals; a semiconductor optical amplifier 13 for receiving the signals from the optical coupler 12 to amplify them and to supply them to an optical band pass filter; and the optical band pass filter 14 receiving the signals from the semiconductor optical amplifier 13 to allow only the RZ clock pattern signals to pass through.
As shown in FIG. 2, in spite of the fact that the input optical intensity is gradually increased, the gain of the semiconductor optical amplifier maintains a constant value up to a certain point, and then, the gain gradually decreases from the certain point.
Now the operation of the cross gain modulation system to which the present invention is applied will be described.
The semiconductor optical amplifier 13 receives two inputs from the optical coupler 12. One of them is the NRZ signal optical data of a wavelength 1. The other is the RZ signal optical clock of a wavelength 2. Here, it is assumed that the NRZ optical data are synchronized to the RZ optical clocks.
The portion (the portion A of FIG. 1) where the NRZ signal pattern "1" of the wavelength 1 is overlapped with the RZ signal clock pattern of the wavelength 2 shows an increased optical intensity. Therefore, in this portion, the gain is not high, and therefore, weak beams are outputted.
On the other hand, the portion (the portion B of FIG. 2) where the NRZ signal pattern "0" of the wavelength 1 is overlapped with the RZ optical clock pattern of the wavelength 2 receives a large gain in the relative terms, and therefore, intense beams are outputted from this portion.
Therefore, the optical band pass filter 14 which is connected to the last terminal of FIG. 1 extracts the beams of the wavelength 2. Under this condition, the RZ signal pattern of the wavelength 2 which has a value opposite to the NRZ signal pattern is outputted.
In the above described conventional technique, the gain characteristics of the semiconductor optical amplifier are utilized, and therefore, the outputted RZ pattern "0" cannot be completely erased. That is, the extinction ratio is not acceptable.
Further, in order to completely erase, the beam intensity of the NRZ signal pattern of the wavelength 1 has to be enormously high.
Further, in the case where the beam intensity of the incoming NRZ signal pattern fluctuates, the beam intensity of the modulated RZ pattern is very much affected.